14-2 Human Chromosomes
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14 –2 Human Chromosomes Sex-Linked Genes
The X chromosome and the Y chromosomes determine sex.
Genes located on these chromosomes are called sex-linked genes .
More than 100 sex-linked genetic disorders have now been mapped to the X chromosome.
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14 –2 Human Chromosomes Sex-Linked Genes
The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
X Chromosome
Duchenne muscular dystrophy
Melanoma
X-inactivation center
X-linked severe combined immunodeficiency (SCID)
Colorblindness
Hemophilia
Y Chromosome
Testis-determining factor
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14 –2 Human Chromosomes Sex-Linked Genes
Why are sex-linked disorders more common in males than in females?
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14 –2 Human Chromosomes Sex-Linked Genes
For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
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14 –2 Human Chromosomes Sex-Linked Genes
Colorblindness
Three human genes associated with color vision are located on the X chromosome.
In males, a defective version of any one of these genes produces colorblindness.
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14 –2 Human Chromosomes Sex-Linked Genes
Possible Inheritance of
Colorblindness Allele
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14 –2 Human Chromosomes Sex-Linked Genes
Hemophilia
The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia.
In hemophilia, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.
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14 –2 Human Chromosomes Sex-Linked Genes
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle.
It is caused by a defective version of the gene that codes for a muscle protein.
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14 –2 Human Chromosomes X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in female cells, one X chromosome is randomly switched off.
This chromosome forms a dense region in the nucleus known as a Barr body.
Barr bodies are generally not found in males because their single X chromosome is still active.
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14 –2 Human Chromosomes Chromosomal Disorders
What problems does nondisjunction cause?
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14 –2 Human Chromosomes Chromosomal Disorders
The most common error in meiosis occurs when homologous chromosomes fail to separate.
This is known as nondisjunction , which means,
“not coming apart.”
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14 –2 Human Chromosomes Chromosomal Disorders
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.
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14 –2 Human Chromosomes Chromosomal Disorders
Nondisjunction
Homologous chromosomes fail to separate.
Meiosis I:
Nondisjunction
Meiosis II
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14 –2 Human Chromosomes Chromosomal Disorders
Down Syndrome
If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome.
Down syndrome involves three copies of chromosome 21.
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14 –2 Human Chromosomes Chromosomal Disorders
Down syndrome produces mild to severe mental retardation.
It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
Down Syndrome Karyotype
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14 –2 Human Chromosomes Chromosomal Disorders
Sex Chromosome Disorders
In females, nondisjunction can lead to Turner’s syndrome.
A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).
Women with Turner’s syndrome are sterile.
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14 –2 Human Chromosomes Chromosomal Disorders
In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,XXY).
The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.
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14 –2
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14 –2
The average human gene consists of how many base pairs of DNA? a. 3000 b. 300 c. 20 d. 30,000
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14 –2
Which of the following genotypes indicates an individual who is a carrier for colorblindness? a. X C X b. X C X c c. X c Y d. X C Y
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14 –2
Colorblindness is much more common in males than in females because a.
the recessive gene on the male’s single X chromosome is expressed.
b. genes on the Y chromosome make genes on the X chromosome more active.
c. females cannot be colorblind.
d. colorblindness is dominant in males and recessive in females.
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14 –2
The presence of a dense region in the nucleus of a cell can be used to determine the a. sex of an individual.
b. blood type of an individual.
c. chromosome number of an individual.
d. genotype of an individual.
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14 –2
Nondisjunction occurs during a. meiosis I.
b. mitosis.
c. meiosis II.
d. between meiosis I and II.
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